i haven't done it yet, but it says in the manual to do it for longer battery life........

To summarize:
1) Charge it and let it sit charged for 2 hours.
2) Discharge it by using it until it shuts down and let it rest 5 hours.

I don't think this will give you "longer battery life," but only accurately display the remaining charge. It is controversial if it extends battery life, but the general consensus is you should calibrate once a month.